Spark plugs of various constructions have been provided for increasing the operating efficiency of an internal combustion engine. Certain prior spark plugs have electrode arrangements for producing a plurality of sparks during each firing cycle of the plug as, for example, the construction shown in J. A. Stahr U.S. Pat. No. 2,208,059, issued July 16, 1940. Other spark plugs have electrodes constructed and arranged to be self-cleaning, as shown in T. T. Bathurst U.S. Pat. No. 1,384,818, issued July 19, 1921. My U.S. Pat. No. 3,872,338, issued Mar. 18, 1975 discloses a spark plug in which the electrodes are encased in a non-conducting material to eliminate electric and chemical corrosion of the electrodes, thereby prolonging the operating life of the spark plug. Still other spark plugs are provided with auxiliary members for modifying the propagation of the combustion flame, as shown in K. Yamazaki U.S. Pat. No. 3,965,384, issued June 22, 1976. While the prior spark plugs provide varying degrees of improved operating results, they do not result in a significant increase in the horsepower output of an engine under given engine operating conditions, and, also, the prior spark plugs do not provide positive ignition of a lean fuel mixture. A spark plug made in accordance with this invention permits the use of a much leaner fuel mixture than is possible with prior spark plugs and without the normally expected decrease in engine horsepower output.